1. Field of the Invention
One aspect of the invention relates to a signal correction device.
2. Description of the Related Art
In apparatuses such as cellular phones or personal computers that perform speech input and output, noise suppressing process for suppressing noise included in the input speech or echo suppressing process for suppressing echo that is generated due to the return of sound from a speaker to a microphone are performed. In the process for suppressing the noise or the echo, various techniques have been proposed (see Japanese Patent No. 3522986, for instance).
In the invention disclosed in Japanese Patent No. 3522986, an orthogonal transform is performed for an input signal, and transform coefficients acquired by performing the orthogonal transform are divided into two groups including a transform coefficient group in which the transform coefficients are included in a lower band than a specific fixed frequency that is determined in consideration of a frequency corresponding to the pitch period of the speech and a transform coefficients group in which the transform coefficients are included in a higher band than the specific fixed frequency. Then, a suppression process is performed for the transform coefficient group in which the transform coefficients are included in the higher band by using suppressing gain (ratio) different for each transform coefficient. On the other hand, the suppression process is performed for the transform coefficient group in which the transform coefficients are included in the lower band by using constant suppressing gain (ratio). Accordingly, even when an orthogonal transform means of a low order number that has a frame length smaller than the pitch period of the speech is used, a distortion is not generated in the speech after noise suppression. Therefore, the computational load relating to the orthogonal transform is light, and degradation of the speech quality does not occur.
However, in a case where the suppression process is performed by using constant suppressing gain (ratio) for a plurality of frequency bands, when the number of the transform coefficient groups (the number of the frequency bands) for which constant suppressing gain (ratio) is used in the same group is too small, rasping musical noise is generated in an interval in which a noise as a non-target signal is included in the input signal. On the other hand, in such a case, when the number of the transform coefficient groups (the number of the frequency bands) for which the constant suppressing gain (ratio) is used in the same group is too large, the distortion of the speech in a speech interval in which a small noise is included may easily increase. Such a problem occurs not only in the noise suppressing process but also in the echo suppressing process. Thus, in a case where echo as an unnecessary non-target signal is inserted into the input signal, when the number of the frequency bands for which a constant ratio is used in the same group is too small, a rasping sound is generated. On the other hand, in such a case, when the number of the frequency bands for which the constant ratio is used in the same group is large, the distortion of the speech increases in an interval in which a small echo is included.
In the invention disclosed in Japanese Patent No. 3522986, the method of dividing the groups is not dynamically changed in accordance with the input signal. Accordingly, even when the noise suppressing process is performed by grouping the transform coefficients that have similar frequency characteristics after the orthogonal transform is performed, a sound which irritates the ear is generated or distortion of the speech increases as described above, depending on the number of the frequency bands for which the constant ratio is used in the same group.